The North: Some sunshine, quite mild.
Wales: Some sunshine, quite mild, but prone to rainy outbursts.
That bit in the middle: Same as the North really.
East Anglia: Nobody cares
The South-East/London: Can be nice and sunny, rain sometimes, but mostly pretty mild. Generally better than anywhere else with the exception of the South-West.
The South-West: The sunniest bit, but still prone to rain and stuff on occasion.

If you've ever been in the Med or anywhere closer than Britain to the equator in summer time, then comparatively, it's mild - top end of the twenties (celcius) would be enough to get fat pale wankers topless in parks.

And it rains. Often. I don't remember this from my childhood, but for the past few years it definitely has.